246 Lamarck's Genera of Shells. 



side of the whorls triplicate, extending from both sides to the cen- 

 tre of the shell, and uniting. Chambers very numerous, small, 

 alternate ; septa imperforate, transverse. 



The nummulites, by a transverse section in the direction of their 

 plane, present from eighteen to twenty-four very narrow whorls, 

 which seem to turn circularly round a central point, yet, never- 

 theless, describe a true spiral line, terminating in the - last whorl ; 

 and since each of them is doubly folded at its exterior margin, they 

 form as many little plates, above and below, as there are whorls, 

 ■which all unite at the two centres. Now, between all these little 

 plates, each whorl of the spiral is divided into a multitude of small 

 chambers, formed by transverse, imperforate septa, extending ra- 

 ther obliquely towards the centre of each disc, losing themselves 

 and disappearing between the plates, as they approach each other. 

 Hence the shell is thickest in the middle. 



Breyn, in 1732, and Jean Gesner, in 1758, conceived the idea 

 that the nummulites are true univalve shells, very analogous to the 

 ammonites, and Bruguieres has since adopted their opinion, of the 

 accuracy of which there can now be little question. They are very 

 common fossils, and extremely abundant in various countries, often 

 forming large stony masses. Bruguieres considers them to be sea 

 shells. 



Type. Nummulites tevigata*. 



Shell lenticular, smooth, slightly convex on both sides. 



Fossil, Villers-Coterets. PI. vi. Fig. 229. 4 Species, all fossil. 

 6. Nautilus f. 



Shell discoidal, spiral, multilocular ; parietes simple, without any 

 suture. Whorls contiguous ; the last enveloping the others. 

 Chambers numerous, narrow, transverse, formed by transverse 

 septa ; last chamber very large ; septa concave on the side next to 

 the aperture, their discs perforated by a tube, and their margins 

 very simple. 



The nautilus is generally rather a large shell, whose spire turns 

 orbicularly in the same plane, round the central summit. A por- 



* Smootb, t Tbe original name, from nauta, a sailor. 



